Friday, May 31, 2013

Employing present, proven technology manned space travel to Mars exceeds NASA’s own limits on astronaut radiation exposure. That limit is calculated in terms of risk of “Radiation Exposure Induced Death,” or “REID,” over an individual astronaut’s life expectancy.

Ironically, as astronauts age their risk of eventually dying from causes unrelated to radiation exposure steadily increase. It’s the kind of risk coldly calculated by insurance providers. Though dying of undiagnosed heart disease is fed into the calculus, such other threats to the older astronaut's long-term survival overshadow their cumulative risk of REID.

None of this is news. This fly in the ointment in need of being overcome before humans can safely experience long-duration spaceflight beyond Earth’s magnetic field was starkly spelled out in the influential “ (2007),” a report put together by the National Academy of Science before the Constellation program was cancelled. The hard numbers have been gathered from the opening of the Space Age, from Explorer 1 through Apollo, from the Voyagers through the International Space Station.

Now these projections have been verified again by an instrument that traveled to Mars with Curiosity.

The lead investigators for these sensors announced their results during a NASA audio press conference Thursday. Dr. Cary Zeitlin, a principal scientist in the Southwest Research Institute’s (SwRI) Space Science and Engineering Division discussed detailed measurements of energetic and highly-ionizing particle radiation gathered during the 253 day, 560 million km journey to deliver the Mars Science Laboratory (MSL) “Curiosity” rover to the floor of Gail crater on Mars.

The Radiation Assessment Detector (RAD) made detailed measurements of the energetic particle radiation environment inside the spacecraft, providing important insights for future human missions to Mars.

NASA/JPL/SwRI

"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Dr. Cary Zeitlin, a principal scientist in SwRI's Space Science and Engineering Division and lead author of Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory, scheduled for publication in the journal Science on May 31.

"Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions," Zeitlin said. "Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself."

Titanium alloy in the hull of a manned spacecraft is a good shield
against most solar particle events, but counter-productive against
the heaviest cosmic rays. These heavy nucleons split and shower
damage into human tissue.

Two forms of radiation pose potential health risks to astronauts in deep space: a chronic low dose of galactic cosmic rays (GCRs) and the possibility of short-term exposures to the solar energetic particles (SEPs) associated with solar flares and coronal mass ejections. Radiation dose is measured in units of Sievert (Sv) or milliSievert (1/1000 Sv). Long-term population studies have shown that exposure to radiation increases a person's lifetime cancer risk; exposure to a dose of 1 Sv is associated with a 5 percent increase in fatal cancer risk.

GCRs tend to be highly energetic, highly penetrating particles that are not stopped by the modest shielding provided by a typical spacecraft. These high-energy particles include a small percentage of so-called heavy ions, which are atomic nuclei without their usual complement of electrons. Heavy ions are known to cause more biological damage than other types of particles.

The solar particles of concern for astronaut safety are typically protons with kinetic energies up to a few hundred MeV (one MeV is a million electron volts). Solar events typically produce very large fluxes of these particles, as well as helium and heavier ions, but rarely produce higher-energy fluxes similar to GCRs. The comparatively low energy of typical SEPs means that spacecraft shielding is much more effective against SEPs than GCRs.

"A vehicle carrying humans into deep space would likely have a 'storm shelter' to protect against solar particles. But the GCRs are harder to stop and, even an aluminum hull a foot thick wouldn't change the dose very much," said Zeitlin.

"The RAD data show an average GCR dose equivalent rate of 1.8 milliSieverts per day in cruise. The total during just the transit phases of a Mars mission would be approximately .66 Sv for a round trip with current propulsion systems," said Zeitlin. Time spent on the surface of Mars might add considerably to the total dose equivalent, depending on shielding conditions and the duration of the stay. Exposure values that ensure crews will not exceed the various space agencies standards are less than 1 Sv.

"Scientists need to validate theories and models with actual measurements, which RAD is now providing. These measurements will be used to better understand how radiation travels through deep space and how it is affected and changed by the spacecraft structure itself," says Donald M. Hassler, a program director at Southwest Research Institute and principal investigator of the RAD investigation. "The spacecraft protects somewhat against lower energy particles, but others can propagate through the structure unchanged or break down into secondary particles."

Only about 5 percent of the radiation dose was associated with solar particles, both because it was a relatively quiet period in the solar cycle and due to shielding provided by the spacecraft. Crew exposures during a human mission back and forth to Mars would depend on the habitat shielding and the unpredictable nature of large SEP events. Even so, the results are representative of a trip to Mars under conditions of low to moderate solar activity.

"This issue will have to be addressed, one way or another, before humans can go into deep space for months or years at a time," said Zeitlin.

SwRI, together with Christian Albrechts University in Kiel, Germany, built RAD with funding from the NASA Human Exploration and Operations Mission Directorate and Germany's national aerospace research center, DLR.

Thursday, May 30, 2013

The Moon's elusive, uneven gravity is clearly seen in this Free-Air Gravity map produced from data returned in 2012 by the twin GRAIL orbiters. Mare Imbrium, for example, at upper right presents a significant anomalous profile, concentrated near what may have been the original "transitory" crater boundary but equally reduced from the lunar average (blue) between that boundary and the outer reaches of Imbrium's present boundary [NASA/JPL/MIT].

Pasadena -- Investigators combing through the huge treasure trove of data returned to Earth by NASA's GRAIL (Gravity Recovery and Interior Laboratory) twin spacecraft Ebb and Flow in 2012 claim to have "uncovered the origin of massive invisible regions that make the moon's gravity uneven, a phenomenon affecting the stability and longevity of lunar-orbiting spacecraft," JPL announced Thursday.

"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now," said Jay Melosh, a GRAIL co-investigator at Purdue University in West Lafayette, Ind., and lead author of the new research. "We believe the data from GRAIL show how the moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies that we recognize as mascons."

The origin of lunar mascons has been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Researchers generally agree mascons resulted from ancient impacts billions of years ago. It was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust.

On a map of the moon's gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact. The increase in density and gravitational pull at a mascon's bulls-eye is caused by lunar material melted from the heat of a long-ago asteroid impact.

"Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts," Melosh said. "Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits."

"Mascons also have been identified in association with impact basins on Mars and Mercury," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "Understanding them on the moon tells us how the largest impacts modified early planetary crusts."

Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow, operated in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometers) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface.

We invite you to take a close look at this sinuous rille in Jules Verne crater on the lunar farside.

Why do we see it approach this portion of an ancient, mare-flooded crater rim and suddenly terminate?

Any flowing river of molten rock (the process responsible for most sinuous rilles) would skirt the base of such a positive-relief structure once encountered ... unless the crater rim formed after the rille. But if the crater is flooded by mare basalts (see context image below) the crater must have formed before the rille, which established itself during mare emplacement.

Simple contextual seven kilometers wide field of view shows a wide distribution of debris aprons bordering nearly every contact zone in the vicinity of this ghost crater on the floor of Jules Verne. View a much larger rendition HERE. LROC NAC mosaic M192002047LR, orbit 13328, May 18, 2012; 66.47° angle of incidence, resolution 0.72 meters per pixel from 70.65 km [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) mosaic covering a 100 km wide field of view, including the western interior of Jules Verne [NASA/GSFC/Arizona State University].

A clue may be present within the rille itself. Note the sloping wall of debris entering the rille at the point where it encounters the crater wall near the center of the Featured Image. This is accumulated debris, which has been shed from the crater rim. If you look closely at the full NAC frame, HERE, you can also see a subtle break in slope around the perimeter of the crater wall that betrays the presence of a debris apron or pediment.

This eroded material may have buried other portions of rille that might indeed have skirted the original rim, giving the visible portion an appearance of protruding out of the rim at a sharp angle. Can you find any additional clues that would help solve the puzzle?

Wednesday, May 29, 2013

A sharp reflectance contrast is found in an unnamed crater on the east floor of Humboldt (26.593°S; 83.764°E). NAC frame M182974061L, illumination is from the west, north is up, image is ~850m wide [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Why do we see such a sharp contrast in reflectance in the above Featured Image between the crater wall on the left and the crater floor on the right? This is the floor and wall of an ~8.5-km diameter, unnamed impact crater within the much larger Humboldt crater on the lunar farside.

The reason for the striking dichotomy in terrain appearance is not a mystery -- it has to do with solar illumination angle.

Our star, the Sun is shining from the west at an elevation above the lunar horizon (~25°) that is close to the slope of the western crater wall. Thus much of the wall on this side is receiving low-angle sunlight and appears relatively dark compared to the crater floor surface, which is more directly illuminated. The floor makes a clean contact with the crater walls, and becomes a zone of deposition for boulders rolling down the slope, including one larger and prominent boulder at the southern end of the frame.

Contextual view of the unnamed crater of interest (center) on the far east floor of Humboldt, its interior and floor highly illuminated by a comparatively low angle of solar illumination (a high Sun). From the global albedo lunar photograph mosaic swept up over the mission of China's lunar orbiter Chang'E-2 [CNSA/CLEP]..

For reasons like this, planetary scientists need to be careful when making interpretations of surface features, and must often use several images collected under a variety of lighting geometries to understand the geomorphology.

The entirety of Humboldt crater, from the early LROC GLD100 Wide Angle Camera (WAC) global mosaic. The unnamed crater of interest is designated [NASA/GSFC/Arizona State University].

The WAC mosaic presents the exotic landscape of Humboldt crater for context. Click HERE to see the full NAC frame.

Accuracy in scientific reporting (and thus the education of the public) is wholly dependent on a reporter’s understanding of the material they’re covering. Making a reporter’s job even more challenging is the fact that some research results themselves can be misleading.

To briefly set the stage on this new work, we believe that the vast majority of craters on the Moon and planets are formed by the collision of solid objects with these bodies. These impacts occur at very high speeds; on the Moon, the average velocity of impact is about 20,000 meters per second. At such speeds, geological materials will vaporize and the mechanics of the formation of a crater are complex. These results have been painstakingly described through laboratory and field studies of both natural and artificial impact craters of a wide range of sizes.

Because we needed to fully understand the mechanics of impact cratering to understand the record in the Apollo lunar samples, much work was conducted toward characterizing the physical and chemical effects of impact on typical rocks. Because impact velocities are typically high, there is little preservation of the projectile in impact craters. Most of the impactor is vaporized and this super-hot silicate vapor is partly lost to space and partly incorporated into the shock melted rocks of the crater interior.

"For every problem there is a solution that is simple, elegant and wrong." - Mencken

The soils returned from the Apollo missions contained a recognizable fraction of material that must have been added by the impacting objects that created its craters. In most soils, this fraction is on the order of a few weight percent. Interestingly, this “meteoritic component” tends to be defined chemically and actual fragments of meteorite in the lunar soil are extremely rare. This observation would seem to support the notion that most of the impacting debris is vaporized at impact and does not occur as fragments on the surface.

However, the speed of impacting projectiles cited above is an average speed, meaning that while some impacts occur at higher velocities, others must occur at lower speeds. As the encounter velocity decreases, there is an increasing likelihood that some portions of the impacting fragments might be preserved on the surface. It is this last possibility that the new paper considers. The authors modeled the effects of the impact of a relatively slow-moving body with the Moon and found that more fragments of the object are preserved than in high velocity impacts. Moreover, by tracing the paths of impactor particles during cratering flow, they find that much of this preserved material ends up on or near the central peak of the resulting crater.

That last finding is interesting because in remote sensing studies of the lunar surface, it is in the central peaks where we find “unusual” compositions, in the sense that those compositions are different from the average upper lunar surface. The traditional explanation for this relation is that because central peaks are derived from well below the impact target, they are exposing deep-seated compositions (lower levels of the crust of the Moon contain different rock types than occur on the surface). The study’s new interpretation suggests instead that the central peaks are covered in debris from the impacting projectile.

One problem with this interpretation is that the “debris covering” of central peaks should occur in a distinct minority of craters (i.e., those created by low velocity impacts). But the exposure of unusual compositions within central peaks of lunar craters is quite common and occurs globally. Moreover, there are as many impacts at higher velocity as at lower velocity. Yet slow impacts would produce less total volume of impact melt and most of the central peak craters on the Moon have abundant melt deposits.

Central peaks of the farside landmark crater Tsiolkovskiy, from over 200 km to the west of the highest promontory. The large, mare-inundated impact crater was very unlikely to have been formed by a "low-velocity" collision. A highly reduced in scale crop from a LROC NAC mosaic, M1098059280, orbit 14176, July 27, 2012; resolution between 4.6 and 5.3 (background) meters, from 87.66 km over 20.44°S, 121.42°E. Enlargement, HERE [NASA/GSFC/Arizona State University].

The most serious flaw in the new study is the assumption that the “unusual minerals,” olivine and spinel (found in many central peaks), are rare on the Moon. They are not rare; although spinel is somewhat sparse on the lunar surface (requiring high pressure for its formation), it has been described as present in lunar rocks from the first sample return and more recently has been found in remote sensing data of impact basin deposits.

In short, there is no compelling reason to believe that the central peaks of many lunar craters are dusted with exotic minerals from asteroids, although such a possibility is certainly not excluded. The minerals that we see in central peaks are all indigenous to the Moon and in some cases, abundant in the lunar crust.

Computer modeling in science has both value and pitfalls. An impact event is extremely messy and complicated. Simultaneously, gigantic shock pressures and temperatures occur, putting billions of particles in motion. Computers are good at keeping track of these particles and the codes developed to model complex, multi-variable phenomena have been shown to at least partly describe the behavior of crater formation on Earth. However, the results of computer models must be interpreted cautiously; small changes in input variables or the conditions of the simulation sometimes result in drastic changes in the output of the model. In addition, there is a tendency in science to believe in numbers, regardless of their provenance. Because a model holds together does not mean that it describes reality.

In science, it is dangerous to embrace a model because it “works” (i.e., comes to closure). Much of the current fracas over human-induced climate change comes from those who contend that the results of computer models constitute “settled science” (whatever that is). Because the computer models say that it may happen, people assume (and some journalists report) that it is happening. In actual fact, we have no direct observational evidence that human-caused emissions of carbon dioxide are causing the climate to change. That conclusion comes from computer models that “show” (project) that humanity’s introduction of “excess” carbon dioxide into the atmosphere by industrialization will increase the magnitude of the greenhouse effect and raise the mean global temperature. But climate (like impact) is a complex, chaotic phenomenon and we still do not fully understand how the Earth’s atmosphere interacts with itself and the cosmos.

In questions of complex natural processes, beware of accepting the results of computer modeling too easily. Computer models are useful tools, but the old software adage of “garbage in, garbage out” still applies. Be familiar with whom and from where the information comes, understand how it is processed and then carefully consider the likelihood of reported accounts.

Tuesday, May 28, 2013

A chain of impact features provides a picturesque tableau (48.659°N; 103.299°E). LROC Narrow Angle Camera (NAC) frame M18286833R, LRO orbit 12051, February 3, 2012; illumination is from the southwest (angle of incidence 63.07°), north is up, image field of view approximately 2 km across [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Today's Feature Image exhibits a chain of impact features that are so closely spaced as to lose their distinction as separate landforms, producing one continuous feature instead.

An asymmetry in the ejecta pattern can also be seen in the form of filamentary tendrils extending to the north.

Based on this ejecta distribution, the secondary bolides likely came from a southerly or southeasterly direction.

Another, earlier look at the same field of view, at slightly higher resolution. LROC NAC M123901314R, orbit 3393, March 22, 2010; angle of incidence 47.54° at 0.56 meters resolution from 53.96 km [NASA/GSFC/Arizona State University].

The WAC mosaic context image is approximately 83 km wide [NASA/GSFC/Arizona State University].

Zooming out to learn what this source might have been produces no obvious candidates, however. None of the medium-sized craters within 40 km of the featured crater chain appear to be particularly young -- a condition required to explain the chain's fresh appearance. Not until we expand our view even further do likely candidates crop up.

This larger scale WAC mosaic context image is just about 470 km wide [NASA/GSFC/Arizona State University].

But even here nothing unambiguous catches the eye. The secondary impacts could have resulted from one of any number of craters, or perhaps from an impact located even further away. A detailed surface study would be necessary before a definitive link could be made tying this ejecta with its crater of origin. Explore the full NAC frame HERE.

Bonus Context: The location of the field of view shown at high resolution in the LROC Featured Image, released May 29, 2013 in the north farside highlands. The terrain is representative of one of the four recognized lunar material groups, the farside anorthositic highland terrain, or FaHT [NASA/GSFC/SVS].

Sunday, May 26, 2013

Eimmart A is a relatively small (7.34 km), fresh-looking crater that impacted close by the east rim and ejecta of crater Eimmart (23.97°N, 64.8°E, 44.99 km in diameter).The impactor that produced Eimmart A may have initially struck on an odd 'contact point': between where ejecta from Eimmart, at its west, met the lava-flooded floor of Mare Anguis (associated to formation of the Crisium Basin), at its east.As a consequence, a wonderfully, bowl-shaped crater formed where material excavated from it was shot out in every direction - whose signatures, today, mainly shows up particularly as bright rays emanating away from the crater (observable through any amateur-sized telescopes ~ 4-inch or greater).

Close-up of the crater, and its inner walls, displays a 'contrast of sorts'! On the eastern side, we see dry-debris flows of materials that have 'slid' down towards the crater's floor, while on its western side, sheets of melted rock have solidified on the surface, in placeWhy the contrast? Why do dry debris flows on one side of the crater contrast against melts on the other side?

New (above) and slightly older (Google/USGS/JAXA) digital elevation models show the anatomy of the Eimmart A impact on the east rim of Eimmart. The view above derived from the LROC QuickMap WAC/NAC GLD100 DEM, below LROC WAC monochrome image overlaid upon the USGS DEM derived from Japan's Kaguya and the United State's DOD Clementine laser range topography.

An oblique impact scenario might be presumed; where the impactor came in at a low angle from the east; whose main energy then produced a predominance in melting on the westwards (down-range), western-side wall. But such impacts usually result in asymmetric-shaped craters -- like in what we see at crater Proclus, an others.But Eimmart A certainly isn't asymmetric: it is perfectly, circularly-formed from what we would expect by an impactor coming in at a high angle. So what would have produced the 'contrast of sorts'?

The wider region in the vicinity of Eimmert A (above left center), under a higher Sun, shows the wider range of the younger crater's bright ejecta (and the darker material of its interior). From a LROC WAC mosaic stitched from four sequential orbital passes, May 8, 2011 [NASA/GSFC/Arizona State University].

Would it be that the impactor, on impact, encountered a more 'harder' surface (the lavas of Mares Anguis) at its eastern side, and a more 'softer' surface (Eimmart's ejecta) at its western side? If so, each side may have had a different, rock-melt gradient (that is, a difference in melting because of previous, dynamically-altering of material events), which might explain the resultant contrast of dry debris versus melt in Eimmart's walls that we see.

Whatever the scenario, while Eimmart itself is impressive enough in the eyepiece, Eimmart A is really the 'eye-candy', the 'stealer', the one that we should appreciate more - given from what we see in this flyover.

Outcrops of layered mare basalt are visible in the interior wall of Caroline Herschel crater (located at 34.48°N, 328.71°E, in western Mare Imbrium).

The mare basalt layers were exposed during the excavation phase of the impact which created this 13.7 km diameter crater. Some debris from the crater rim and the wall have fallen over the layers but the structure of the outcrop is still preserved.

The crater is superposed on a north-south trending wrinkle ridge which is visible in the LROC WAC context image below. This crater is named after Caroline Herschel, an astronomer and half of the sister/brother science team with astronomer Sir William Herschel.

Caroline discovered several comets, and in 1828 the Royal Astronomical Society awarded her their Gold Medal for her work. She made observations, kept detailed records, performed complex mathematical calculations, and polished her own telescope mirrors. Caroline has multiple comets named after her as well as the lovely lunar crater in today's Featured Image.

LROC Wide Angle Camera (WAC) context for Caroline Hershel. The white asterisk marks the area of basalt layering in the Featured Image. Field of view is 48 kilometers [NASA/GSFC/Arizona State University].

What may be a newly resolved "pit crater," similar to at least three other unique features found elsewhere on the Moon. This one is near the equator in Mare Fecunditatus (0.92°S, 48.66°E). The nearly circular 110 meter-wide opening may or may not narrow in diameter further into its interior. LROC Narrow Angle Camera (NAC) observation M1107960917R, (at 180%) LRO orbit 11562, November 19, 2012; angle of incidence 62.93° from 108 km [NASA/GSFC/Arizona State University].

Joel Raupe

Lunar Pioneer

It’s time to inventory the Moon’s “pit craters.” The first, the "Haruyama," or "Marius Hills pit crater" (14.065°N, 303.224°E) is in a sinuous rille immediately west of the famous shield volcano range in Oceanus Procellarum.

A second and third have now been found and photographed from many angles, in Mare Tranquilitatis (8.34°N, 33.22°E) and Mare Ingenii (35.95°S, 166.06°E), respectively.

It appears the LROC team at Arizona State University uncovered another, over the past year, out on the vast equatorial plains of Mare Fecunditatis (0.92°S, 48.66°E).

This is remarkable for a number of reasons, not least among them the mission’s elapsed time. As of this writing LRO has completed 17,801 orbits around the Moon. Without knowing the exact percentage of the lunar surface yet to be photographed by the LROC Narrow Angle Cameras – it’s remarkable a 110 meter wide target could have been missed until relatively recently.

That is it might seem remarkable, until we consider some basic “beta angles,” so to speak, some basic mission priorities and logistics.

Location of a possible pit crater in a 155 km-wide field of view of northwest Mare Fecunditatis. LROC QuickMap 250 meter per pixel resolution [NASA/GSFC/Arizona State University].

The target is within a single degree south of the equator. Obviously a spacecraft in polar orbit is going to see its orbital pathways and targeting opportunities converge directly over the poles, conversely those same opportunities will be at their greatest distance apart at the equator.

A closer look through one particularly fine set of LROC Wide Angle Camera passes over target (arrow), the feature is just visible in this imperfectly merged monochrome (604 nm) WAC mosaic swept up during three sequential orbital passes; from 47.4 km altitude. Resolution roughly 55 meters, 63° angle of incidence; field of view a little over 40 km, from west to east [NASA/GSFC/Arizona State University].

Secondly, this has got to be one of the Moon’s great “Rub’ al khali's,” an empty quarter, which must have seemed nearly void of inviting targets, with very inviting targets nearby, particularly to the west, where the fascinating Messier and Messier A craters reside. The east rim of Messier B is only a little over 20 kilometers directly to the west. The desire, even the need, to slew the spacecraft and camera’s off nadir to examine these and other nearby targets is reason enough for the pit to have been overlooked.

Then there’s the target itself, which brightens considerably between a 30° and 0° angle of incidence. Under the highest sun, near noon, and again, very near the equator, the target looks like what it may in fact be: an unusual but still rather commonplace nearly fresh crater.

It proves, yet again, that there are still great new discoveries yet to be made on the Moon.

Raw rendition of the LROC NAC observation which may have touched off further interest in a new "target of opportunity," in the months that followed. The pit crater in Mare Fecunditatis shows up on the very edge of this frame from orbit 13087, April 28, 2012. LROC NAC M190280022L, 62.93° angle of incidence, 1.09 meters resolution from 108.01 km [NASA/GSFC/Arizona State University].

Having to guess just what drew their interest, the target seems to have been photographed at high resolution April 28, 2012, in orbit 13087. Amazingly, the pit was nearly missed. You can see the north half of the target at the very top of LROC Observation M190280022L, HERE.

By last fall it seems the pit was directly targeted under three lighting conditions, the first, last September, must have seemed disappointing. With the Sun only five degrees from directly overhead what little topography might be seen on target and in the region may have seemed washed out in shadowless albedo contrasts. If this was a pit crater the “ledge,” if any, was not overshadowing.

First full close-up released to the PDS shows a brightly lit interior, and little to no depth. But the Sun was high, and the location less than a degree south of the equator. LROC NAC M1103245601L, orbit 14902, September 25, 2012, angle of incidence 7.765° at 0.94 meters resolution, from 108.88 km [NASA/GSFC/Arizona State University].

A month later the Sun was a little more favorable. LROC NAC M1105602888L, orbit 15232, October 23, 2012; angle of incidence 35.18° at 0.93 meters resolution from 108.28 km [NASA/GSFC/Arizona State University].

After yet another month, the mid-morning Sun is at an even greater angle. Shown at its original resolution, this is the image at the top of the post. LROC NAC M1107960917R, orbit 15562, November 19, 2012; angle of incidence 62.93° at 1.1 meters resolution, from 108.01 km [NASA/GSFC/Arizona State University].

Wednesday, May 22, 2013

Portion of an unnamed concentric crater in Apollo Basin. Sun is incident from the right to the left. LROC Narrow Angle Camera (NAC) mosaic M1122245918LR, orbit 17571, May 3, 2013; image field of view is 6.3 km [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

The double-arch shape in the Featured Image is a portion of an unnamed concentric crater located in the northwestern extent of Apollo Basin (basin center at 35.687°S, 208.232°E).

The concentric crater has an inner ring, centered on 30.757°S, 205.931°E, a middle ring, and then the crater rim.

The crater formed within the mare basalt that fills Apollo Basin. The formation mechanism for concentric craters like this one is not entirely clear. One theory is that the target material is made of multiple stratigraphic layers with different strengths. If the difference between the strengths of the layers is great enough, the impact may form concentric rings.

In the late 1960s laboratory experiments replicated the concentric shape of craters using targets with loose, granular material over stronger, more cohesive layers. The laboratory experiments use different materials and are at smaller scales than their lunar counterparts. Still, experiments like these are important for comparing what we see on the lunar surface to basic physical principles. What if an impact occurs in an area with highland material as one layer and then mare basalt as a second layer? What crater shape is produced if you introduce a regolith layer? These are the questions that lunar geologists use to design their experiments.

LROC WMS Wide Angle Camera mosaic of the concentric crater in context with north and northwestern Apollo basin, The crater of interest is 11.5 km across [NASA/GSFC/Arizona State University].

Tuesday, May 21, 2013

New Crater - small white blotch, with black center (in center of image), is likely a new impact crater formed during the LRO mission (since June 2009). Inset in lower left, a closer look at a 38 meter wide field of view, a 4x enlargement from unreleased LROC Narrow Angle Camera (NAC) observation M1117799545; full field of view above is 480 meters wide [NASA/GSFC/Arizona State University].

Impacts are the most important force shaping the surfaces of planets, moons, asteroids, and comets. In fact small asteroids and comets impact the Earth and Moon every day. On Earth, most of these small bolides burn up in the atmosphere and can be seen at night as meteors (aka shooting stars). Larger meteoroids hit the surface as fragments, or intact. Recall the recent meteor that exploded over Chelyabinsk Russia, and caused minor, yet widespread damage. That meteor was thought to be 15 to 20 meters in diameter before breaking up, fortunately the atmosphere absorbed most of the energy of that event. More recently in the news, a much smaller object was seen to impact the Moon on 17 March 2013. A team of Marshall Space Flight Center scientists photographed this bright flash (and many others), and then computed latitude and longitude coordinates.

Temporal Comparison - Before and after LROC NAC images showing one of many distinctive white splotches that the LROC SOC team is finding in temporal comparisons [NASA/GSFC/Arizona State University].

The LROC team targets the coordinates of flashes for future observations in hopes of finding a newly formed crater. If an LROC NAC image of the impact area was acquired before the event, it is very likely that the crater can be found by comparing before and after images. If no pre-existing image exists then the task becomes nearly impossible because there are so many small, young impact craters on the Moon! So far the LROC team has found one crater associated with Marshall impact flash reports, certainly more will follow. However, the search is not limited to Marshall targets - in fact the LROC team has found over seventy potential new impact craters that formed since LRO went into orbit.

Why the adjective "potential" in front of "new impact craters"? What we are seeing in most cases are reflectance splotches, some low reflectance and some high reflectance. In only a handful of these observations can a crater be resolved. It is likely in the "other" cases that the crater is simply too small to be resolved in the NAC images, which have pixel sizes ranging from 50 cm to 150 cm. To pick out the crater you need to have at least three to five pixels, depending on lighting conditions. So craters need to be bigger than 1.5 meters (at best) to be resolved. So far, the largest new crater definitively identified in the NAC-to-NAC temporal comparisons is 7 meters (23 feet) in diameter.

Ten changes - LROC NAC image footprint on LROC WAC mosaic background, the red square indicates small (a few meters) changes in the surface over the course of a year [NASA/GSFC/Arizona State University].

In some cases, clusters of new splotches (craters) are seen in the NAC comparisons. What causes the clusters? Perhaps a bolide breaks up due to gravitational forces as it nears the surface? Or maybe these splotches are secondaries formed from a larger nearby primary impact? A search for new craters in the WAC has so far revealed no new larger parent crater, but the search continues.

A great advantage of a long-lived LRO mission is to make such temporal comparisons possible over a statistically significant percentage of the Moon. As the new observations are acquired, more and larger new impacts will be discovered. LROC will image the 17th of March Marshall flash target over the next several opportunities, thus covering the full area (the coordinates are only known to about the width of a NAC pair). If pre-existing NAC images of the impact site exist it should be easy to identify the ejecta in the post-flash image - and perhaps even resolve the crater. Stay tuned!

Closing thought: Similar to the GRAIL impact craters some of the "new" craters may exhibit a low reflectance ejecta blanket. Once again the mysterious Moon turns the table on conventional wisdom!

Friday, May 17, 2013

Impact with kinetic energy equivalent to 5 tons of TNT, March 17, 2013. Still from NASA ScienceCast released May 17. The event was the brightest of recorded over eight year span of NASA lunar impact monitoring program. Video still shows the nearside's earthshine-lit western hemisphere at the moment of impact, in the southern tier of Mare Imbrium, north by northwest of Copernicus [NASA/Science].

For the past 8 years, NASA astronomers have been monitoring the Moon for signs of explosions caused by meteoroids hitting the lunar surface. "Lunar meteor showers" have turned out to be more common than anyone expected, with hundreds of detectable impacts occurring every year.

They've just seen the biggest explosion in the history of the program.

"On March 17, 2013, an object about the size of a small boulder hit the lunar surface in Mare Imbrium," says Bill Cooke of NASA's Meteoroid Environment Office. "It exploded in a flash nearly 10 times as bright as anything we've ever seen before."

Anyone looking at the Moon at the moment of impact could have seen the explosion--no telescope required. For about one second, the impact site was glowing like a 4th magnitude star.

Ron Suggs, an analyst at the Marshall Space Flight Center, was the first to notice the impact in a digital video recorded by one of the monitoring program's 14-inch telescopes. "It jumped right out at me, it was so bright," he recalls.

NASA's lunar monitoring program has detected hundreds of meteoroid impacts over the eight year formal history of the program. The brightest, detected March 17, in Mare Imbrium, is marked by the red square [NASA/Science].

An impact on the trailing eastern limb of the Moon monitored January 8, 2008.

Cooke believes the lunar impact might have been part of a much larger event.

"On the night of March 17, NASA and University of Western Ontario all-sky cameras picked up an unusual number of deep-penetrating meteors right here on Earth," he says. "These fireballs were traveling along nearly identical orbits between Earth and the asteroid belt."

This means Earth and the Moon were pelted by meteoroids at about the same time.

“My working hypothesis is that the two events are related, and that this constitutes a short duration cluster of material encountered by the Earth-Moon system," says Cooke.

One of the goals of the lunar monitoring program is to identify new streams of space debris that pose a potential threat to the Earth-Moon system. The March 17th event seems to be a good candidate.

Controllers of NASA's Lunar Reconnaissance Orbiter have been notified of the strike. The crater could be as wide as 20 meters, which would make it an easy target for LRO the next time the spacecraft passes over the impact site. Comparing the size of the crater to the brightness of the flash would give researchers a valuable "ground truth" measurement to validate lunar impact models.

Unlike Earth, which has an atmosphere to protect it, the Moon is airless and exposed. "Lunar meteors" crash into the ground with fair frequency. Since the monitoring program began in 2005, NASA’s lunar impact team has detected more than 300 strikes, most orders of magnitude fainter than the March 17th event. Statistically speaking, more than half of all lunar meteors come from known meteoroid streams such as the Perseids and Leonids. The rest are sporadic meteors--random bits of comet and asteroid debris of unknown parentage.

U.S. Space Exploration Policy eventually calls for extended astronaut stays on the lunar surface. Identifying the sources of lunar meteors and measuring their impact rates gives future lunar explorers an idea of what to expect. Is it safe to go on a moonwalk, or not? The middle of March might be a good time to stay inside.

"We'll be keeping an eye out for signs of a repeat performance next year when the Earth-Moon system passes through the same region of space," says Cooke. “Meanwhile, our analysis of the March 17th event continues.”